TATB based polymer bonded explosive (PBX) was studied by positron annihilation lifetime spectra (PALS) before and after heat treatment. The effect of heat treatment on the microstructure of TATB based PBX was analyzed. The results show that the component of positron annihilation lifetime τ₂ is related to the micropores in PBX. After heat treatment, τ₂ increases while its intensity decreases significantly. Therefore, the dimension of pores in PBX increases while the number of pores decreases after heat treatment.